Amplifiers or preamplifiers which contain a differential pair of transistors are commonly used as input stages for amplifier circuits, particularly when low noise and input offset are required. To minimize the noise and input offset, large transistors are preferable, but this has the undesirable consequence of increasing the input capacitance of the amplifier.
Differential amplifiers suffer particularly from Miller capacitance, which arises from the fact that the base (input) and collector terminals of the input transistors are pulled in opposite directions by the increased voltage drop across the load as the current through the transistor is varied. That is, the voltage at the collector terminal decreases as the voltage at the base terminal increases, and the voltage at the collector terminal increases as the voltage at the base terminal decreases.
The conventional solution to this problem has been to connect a pair of cascode transistors between the input transistors and the load resistors. While this reduces the effective input capacitance to a value equal to twice the capacitance of the collector/base junction of the input transistor, this level is still too high for certain applications. For example, a lower input capacitance is desirable in a preamplifier used in disk drives. In this application, the dual objectives of low noise and low input capacitance are in direct conflict.
A solution to this problem is very much needed, particularly one that has a simple structure and consumes a minimum of power.